1. Field of the Invention
The present invention relates to a serial digital signal transmission apparatus for a high definition television (HDTV) signal, and more particularly to a transmission apparatus for transmitting serial digital interface signals for high definition television signal (HD-SDI; High Definition Television Serial Digital Interface).
2. Description of the Related Art
As a system of transmitting continuous bit rate (CBR) signals asynchronous with a network clock in a synchronous digital hierarchy (SDH) network, a pulse stuffing method is known. This pulse stuffing method is technique to adjust bit intervals to make it constant by inserting extra bits (stuffing bits).
On the other hand, a method of transmitting continuous bit rate (CBR) signals asynchronous with a network clock in an asynchronous transfer mode (ATM) network is prescribed in the Recommendation I.363.1 (B-ISDN ATM Adaptation Layer Specification: Type 1 AAL) of the ITU Telecommunication Standardization Sector (ITU-T). This specification prescribes (in 2.5.2.2.2 (Synchronous Residual Time Stamp (SRTS) method), p. 14) with respect to CBR signals a synchronous residual time stamp (SRTS) method whereby a residual time stamp value (RTS value), which is the difference between the respective least significant four bits of the actual count of the system clock while the CBR clock is counted a given number of times and the predictable count, is transferred from the transmitting side to the receiving side to reduce the quantity of information to be transmitted because, in an asynchronous transfer in which signals asynchronous with the network clock are sent from the transmitting side, the reproduction of the clock frequency on the transmitting side is needed on the receiving side.
However, this pulse stuffing method cannot be applied where CBR signals are high speed signals, such as HDTV serial digital signals (HD-SDI) of 1.485 Gb/s. Nor can the synchronous residual time stamp (SRTS) method by which CBR signals asynchronous with a network clock are transmitted in an ATM network be applied as it is to high speed CBR signals, such as HDTV serial digital signals (HD-SDI) whose bit rate is 1.485 Gb/s, because it is prescribed for network clocks of 155.52 MHz (STM-1).
The SRTS method has another problem that, since it involves many parameters that can be determined as the user likes, the jitter of signals on the output (receiving) side will become great and output signals can serve no practical purpose unless optimal parameters are chosen.